1. Field of the Invention
The present invention relates to electronic devices including a wiring substrate and at least two units provided thereon, and more particularly, to an improvement in the strength for fixing a metal case and surface mount electronic components to the wiring substrate or for fixing a plurality of surface mount electronic components thereto.
2. Description of the Related Art
High-frequency electronic devices, such as RF devices for mobile phones and wireless communication, include a wiring substrate and surface mount electronic components mounted thereon via soldering.
FIG. 8 is a schematic cross-sectional view of a related electronic device 101, which is of interest to the present invention.
The electronic device 101 is a hybrid electronic device including a wiring substrate 102. The wiring substrate 102 is a multilayer ceramic substrate. Although not shown in the figure, on end surfaces of the periphery or the bottom surface of the wiring substrate 102, several terminal electrodes are provided, and on the top surface of the wiring substrate 102, several conductive lands are provided.
At least two surface mount electronic components 103 and 104 are mounted on the top surface of the wiring substrate 102 by soldering. One electronic component 103 is an IC chip component and is soldered to the conductive lands provided on the top surface of the wiring substrate 102 with several solder bumps 105 provided therebetween. The other electronic component 104 is a chip component, such as a capacitor, inductor, or quartz oscillator, and is soldered to the conductive lands provided on the top surface of the wiring substrate 102 with solder fillets 106 formed via soldering.
In the electronic device 101 having the structure described above, a case (not shown) may be fixed to the wiring substrate 102 so as to cover the electronic components 103 and 104.
However, the electronic device 101 shown in FIG. 8 has the following problems.
First, when impact is applied to the wiring substrate 102 due to a fall, bending, or other external force, at least one of the electronic components 103 and 104 mounted on the wiring substrate 102 may fall therefrom or be damaged.
In addition, when the size of the electronic component mounted on the wiring substrate 102 is increased, such as an IC chip component, or when the number of electronic components mounted on the wiring substrate 102 is increased, the size of the wiring substrate 102 must be increased. In addition, since the electronic device 101 has been increasingly required to have a smaller height, the thickness of the wiring substrate 102 must be decreased. Accordingly, damage, such as breakage, caused by external forces are more likely to occur in the wiring substrate 102.
In addition, when the solder bumps 105 provided for the electronic component 103 are formed of eutectic solder, when the electronic device 101 is mounted as a module component on an appropriate motherboard (not shown) via a solder reflow process or other suitable soldering method, the temperature applied when the device is mounted may be higher than that at which the eutectic solder is melted in many cases. In this case, when the solder bumps 105 are melted, the electronic component 103 may move with respect to the wiring substrate 102, and in the worst case, electrical short circuiting caused by the solder will occur between the adjacent conductive lands which are to be connected to the respective solder bumps 105, thereby producing electronic devices 101 which must be rejected.
In addition, in an electronic device provided with a case, when the case is made, for example, of a metal, soldering is frequently performed to fix the case to the wiring substrate. However, due to insufficient strength obtained by soldering, in some cases, the case falls off or is removed from the wiring substrate.
A technique which is of interest to the present invention is disclosed in Japanese Unexamined Patent Application Publication Nos. 5-47968 and 8-17853 in which a resin is provided in the case so as to contact the wiring substrate, and the electronic component thereon is covered with the resin. Since the resin is in contact with the case, the resin serves as an adhesive for adhering the case and the wiring substrate to each other, and hence, the strength of fixing the case to the wiring substrate is increased by the resin.
However, in the structure disclosed in Japanese Unexamined Patent Application Publication Nos. 5-47968 and 8-17853, since the resin covers the electronic component, the solder applied for mounting the electronic component on the wiring substrate is also covered with the resin, and the structure described above may cause the following problems.
That is, the electronic device described above is mounted on a mounting substrate such as a motherboard so that the wiring substrate is brought into contact therewith. In this step, when a solder reflow process is used, the temperature for solder reflow may be higher than the melting point of the solder used for mounting the electronic component to the wiring substrate. In this case, since the solder used for mounting the electronic component is expanded by melting, or volatile components of the solder is expanded, stress is applied to the resin covering the electronic component, the adhesion strength between the resin and the wiring substrate is decreased, and as a result, interface separation therebetween may occur.
Accordingly, when interface separation occurs, the melted solder flows in the portion at which the interface separation occurs, and as a result, the solder causes short circuiting between different terminal electrodes of the electronic component mounted on the wiring substrate, resulting in short-circuiting defects.
The short circuiting defects described above are more likely to occur when the resin is arranged in the case so as to cover the electronic component, and when the resin is not provided in the case, the defects might be prevented. However, when the resin is not provided in the case as described above, the fixing strength of the case to the wiring substrate is reduced.
To overcome the problems described above, preferred embodiments of the present invention provide an electronic device which increases the fixing strength of an electronic component to a wiring substrate, reduces the occurrences of damage thereto, and prevents undesirable movement of the electronic component in a solder reflow process.
In addition, the electronic device according to preferred embodiments of the present invention prevents short circuiting defects caused by a resin provided in a case while increasing the fixing strength of a case to the wiring substrate.
An electronic device according to a preferred embodiment of the present invention includes a wiring substrate, a first unit and a second unit which are mounted on the wiring substrate, and an adhesive resin which adheres the first unit to the second unit, wherein the adhesive resin is not in contact with the wiring substrate. Accordingly, the adhesive resin mechanically integrates the first unit with the second units via adhesion.
The first unit may preferably include at least one surface mount electronic component which is mounted on the wiring substrate via soldering, the second unit preferably includes a case fixed to the wiring substrate so as to cover the electronic component. The adhesive resin is preferably provided between the top surface of the electronic component and the bottom surface of the top wall of the case.
The adhesive resin increases the fixing strength of the case to the wiring substrate with the electronic component provided therebetween. In addition, since the adhesive resin is arranged so as not to contact the wiring substrate, interface separation between the wiring substrate and the resin is prevented even when solder used for mounting the electronic component on the wiring substrate is melted and expanded, and since the solder does not flow in the portion at which the interface separation occurs, short circuiting is prevented. That is, the adhesive resin is preferably arranged so as not to be in contact with solder for connecting the surface mount electronic component or the case to the wiring substrate.
After the electronic component is mounted, in order to easily supply the adhesive resin between the electronic component and the case, an opening is preferably provided in the top wall of the case for supplying the adhesive resin from the outside.
To prevent foreign materials from intruding into the case through this opening, and to avoid limiting a suction position to which a suction nozzle of a mounting machine is applied in a mounting step, the opening is preferably sealed with the adhesive resin.
In addition, to prevent interference with marking on the top wall of the case, the opening is preferably provided at a position spaced from the center of the top wall of the case toward the peripheral side thereof.
In the electronic device of preferred embodiments of the present invention, when the first unit includes a plurality of the surface mount electronic components, the adhesive resin is provided across at least the plurality of electronic components.
When the top surface of the electronic component has at least a resin portion, and the adhesive resin is provided on resin portion for adhesion, the adhesion strength of the adhesive resin is further increased.
In preferred embodiments of the present invention, the case is preferably made of metal. In this case, the case is fixed to the wiring substrate via soldering.
The electronic device of preferred embodiments of the present invention preferably further includes a third unit disposed between a sidewall of the case and the wiring substrate. In this case, even when the position of the case fixed to the wiring substrate is shifted, to prevent the case from damaging the third unit (in particular, surface mount electronic component), the sidewall of the case and the third unit are preferably spaced from one another.
In the electronic device of preferred embodiments of the present invention, when the first unit includes a plurality of electronic components, and the plurality of electronic components has top surfaces having different areas from each other, the adhesive resin is preferably provided on at least the top surface of the electronic component having the largest top surface area. This increases the adhesion area of the adhesive resin to the electronic component, and the adhesive resin does not undesirably flow down to the wiring substrate.
In addition, when the first unit includes a plurality of electronic components, and the plurality of electronic components have different heights from each other, the adhesive resin is preferably provided on at least the top surface of the electronic component having the greatest height. This decreases the amount of the adhesive resin required to adhere the electronic component to the case, and the adhesive resin does not undesirably flow down to the wiring substrate.
In addition, in the electronic device of preferred embodiments of the present invention, when the electronic component includes a first and a second electronic component, and the adhesive resin includes a first adhesive resin for adhering the first electronic component to the case and a second adhesive resin for adhering the first electronic component to the second electronic component, the first adhesive resin and the second adhesive resin may have different compositions. In an uncured state, the first adhesive resin preferably has a low viscosity and a low thixotropic index as compared to that of the second adhesive resin. The reasons for this are that the first adhesive resin is likely to spread along the top surface of the first electronic component, and the second adhesive resin is unlikely to flow down to the wiring substrate.
In another preferred embodiment of the present invention, the first unit includes a first surface mount electronic component, the second unit includes a second surface mount electronic component, and the first and the second surface mount electronic components are mounted on the wiring substrate via soldering, and hence, the electronic device of this preferred embodiment of the present invention defines a hybrid electronic component. The adhesive resin mechanically integrates the first electronic component with the second electronic component via adhesion.
As described above, one of the first electronic component and the second electronic component adhered to each other via the adhesive resin preferably has the largest planar area of the electronic components mounted on the wiring substrate. The electronic component having the largest planar area may be an IC chip component.
As described above, when the adhesive resin is provided on the electronic component having the largest planar area, the other electronic component adhered thereto with the adhesive resin described above preferably has the second largest planar area of the electronic components mounted on the wiring substrate.
In the electronic device of preferred embodiments of the present invention, when a third surface mount electronic component is further mounted on the wiring substrate via soldering, and the second and the third electronic components are provided at two opposing sides of the first electronic component opposing each other, the first electronic component is preferably adhered to the second and the third electronic components with the adhesive resin provided therebetween.
In the second preferred embodiment of the present invention, a case fixed to the wiring substrate is further provided so as to cover the electronic components. In the case described above, the adhesive resin is also preferably adhered to the case. In addition, on the top wall of the case, an opening is preferably provided for supplying the adhesive resin from the outside.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.